Something you say to show someone they lack presence.

This comes from the anime Yuru Yuri.
Sometimes, when you see Akari, the sound effect "Akarin" is played. Then she disappears leaving behind not much more than a dotted outline.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.

A woman who is a mom, an think she is entitled to everything she wants, she will most properly yell at you to bend your will. If you encounter "A Karin" then just keep in mind that her parents dropped her down the stairs as a child, but watch out "A Karin" can be violent. She has the ability to not wear facemasks and yell till your ears bleed.